Gas delivery system

ABSTRACT

A gas delivery system for gas fuelled internal combustion engines, comprising a gas fuel delivery means for delivering a controlled amount of gaseous fuel to a region adjacent a source of ignition. The gas fuel delivery means comprises first and second delivery means for delivering gaseous fuel to a pre-combustion zone and a combustion zone respectively. Gas control means control the relative proportions of gaseous fuel delivered by the first and second delivery means respectively so that combustion in the combustion zone can be achieved with minimum gaseous fuel by initiating ignition of gaseous fuel in the pre-combustion zone. Thus, the fuel/air ratio within the combustion zone can be minimized (lean burn regime) without the onset of engine misfire.

FIELD OF THE INVENTION

This invention relates to a gas delivery system for an internalcombustion engine and more particularly, but not exclusively, to a sparkignition engine which operates on gaseous fuel. In one arrangement thisinvention relates to a spark ignition engine converted from a dieselfuelled or compression ignition engine. In another arrangement thisinvention relates to a dual fuel engine operating as a compressionignition engine. In such an engine operating in the dual fuel mode, itis common for a gaseous fuel to be mixed with the air before inductionof the air into the engine whilst reducing the amount of dieselinjected. In this specification the term "compression ignition engine"is intended to refer not only to an engine operating on a constantpressure, that is a diesel cycle, but also to an engine operating on acompression ignition cycle.

BACKGROUND TO THE INVENTION

When operating an engine with a gaseous fuel it is a well known practiceto introduce the gaseous fuel with the inlet air to the cylinder duringthe air inlet stroke, thus causing a relatively homogenous mixture ofgas and air during the combustion stroke. It is advantageous to reducethe fuel/air ratio to a minimum value in the combustion of gas as afuel, as this reduces the carbon monoxide and hydrocarbon content of theexhaust and enhances fuel efficiencies. If the ratio is reducedsufficiently beyond the stoichiometric value a substantial reduction inthe nitrous oxide content of the exhaust gas may also be achieved. Thisis commonly called a lean burn regime. With current gas engines thelimiting factor in the reduction of the ratio to achieve lean burn isthat which occurs when the average ratio of fuel to air is lowered to alevel where the density of the fuel is such as to prevent initiation offuel ignition, and thus the engine misfires.

SUMMARY OF THE INVENTION

The present invention was developed with a view to providing a method ofgas delivery and a gas delivery system for a gas fuelled internalcombustion engine wherein the fuel/air ratio can be minimised withoutthe onset of misfire of the engine.

According to the present invention there is provided a gas deliverysystem for a gas fuelled internal combustion engine, the systemcomprising:

gas fuel delivery means for delivering a controlled amount of gaseousfuel to a region adjacent a source of ignition, said region comprising apre-combustion zone located in a separate pre-combustion chamber whichis located in immediate proximity to said ignition source and which isin direct communication through an orifice with a combustion zonelocated in a cylinder of the engine, said gas fuel delivery meanscomprising first and second gas delivery lines for delivering acontrolled amount of gaseous fuel to said pre-combustion zone andcombustion zone respectively from a common gas injector, and furthercomprising gas control means for controlling the relative proportions ofgaseous fuel delivered by said first and second delivery meansrespectively whereby, in use, combustion in said combustion zone can beachieved with minimum gaseous fuel by initiating ignition of gaseousfuel in said pre-combustion zone.

In this specification the pre-combustion zone is that zone within whichit is desirous to initiate the combustion process, and the combustionzone is that region to which the resultant effects of gaseous fuelignition in the pre-combustion zone are directed to achieve combustionof the remaining gaseous fuel.

Advantageously said gas control means comprises a gas flow valve forcontrolling the quantity of gaseous fuel delivered to saidpre-combustion zone and/or combustion zone. More particularly, said gasflow valve may be a one way valve for controlling the quantity ofgaseous fuel delivered via said first gas delivery line to saidpre-combustion chamber.

Typically the relative proportions of gaseous fuel delivered by saidfirst and second delivery means is fixed for a particular engine. Therelative proportion of gaseous fuel delivered by said first deliverymeans may be in the range of 1 to 10 percent of the total quantity ofgaseous fuel delivered to said region by said gas fuel delivery means.

In one embodiment said gas control means is provided with a first inputto enable the control means to control the supply of gaseous fuel at apredetermined rate to the pre-combustion zone and/or the combustion zonein accordance with at least one operating parameter. The gas controlmeans may be provided with a second input derived from a feedback signalindicative of the amount of gaseous fuel injected into thepre-combustion and combustion zones, and the control means being adaptedto adjust the supply of gaseous fuel whilst still responding to thatsignal by the first input.

In one embodiment the gas fuel delivery means may include continuousflow valves and the gas control means controls the proportion of gaseousfuel being delivered to the pre-combustion zone and the combustion zonerespectively, as well as the total quantity of gas being supplied overtime to the engine as a whole. In another embodiment the gas fueldelivery means may include incremental flow valves.

In relation to the previously mentioned embodiments, said at least oneoperating parameter of the engine may comprise the engine speed, or theposition of the speed control (that is, the throttle) of the engine,alone or in combination with the engine speed. Further examples of saidat least one operating parameter of the engine may include singularly orin conjunction with any one or more of the following parameters: airsupply temperature; air supply pressure; gas supply temperature; gassupply pressure; engine phase; dynamic engine mode determination; and,battery voltage.

In relation to the previously mentioned embodiments, in one embodimentthe feedback signal is derived from a measurement directly or indirectlyof the exhaust gas contents of the engine for the presence of carbonmonoxides, hydrocarbons, nitrous oxides or other such unwantedemissions. One such measurement may be derived from a commonly availablelambda sensor which monitors the oxygen content of the exhaust gasstream. In a still further embodiment instead of a measurement of theexhaust gas contents a feedback signal is derived from a measurement ofthe onset of misfire in the engine either indirectly or directly.

Optimal control of the relative proportions and quantity of the flow ofgaseous fuel to the pre-combustion zone and the combustion zone can thusbe determined, in conjunction with test bed testing to give goodignition with the minimum amount of gaseous fuel. After initialcalibration, it is not normally necessary to re-calibrate the engine. Itwill be appreciated that as the engine is used, the amount of gaseousfuel fed to the engine by the gas delivery system may be varied. Byproviding a feed-back signal as described, it may be assured that theamount of fuel delivered for the given operating conditions of theengine is optimised.

According to another aspect of the present invention there is provided amethod of delivering gas for a gas fuelled internal combustion engine,the method comprising:

delivering a controlled amount of gaseous fuel to a region adjacent asource of ignition, said region comprising a pre-combustion zone and acombustion zone in communication with each other;

controlling the relative proportions of gaseous fuel delivered to saidpre-combustion zone and combustion zone respectively whereby, in use,combustion in said combustion zone can be achieved with minimum gaseousfuel by initiating ignition of gaseous fuel in said pre-combustion zone.

Preferably, the method further comprises delivering a controlled amountof air to the combustion zone to maintain a minimum air to fuel ratio insaid combustion zone, some of the air/fuel mixture in said combustionzone being communicated to said pre-combustion zone during initiation ofignition of gaseous fuel in said pre-combustion zone.

Typically the method also includes controlling the total volume ofgaseous fuel delivered to the engine over time.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a better understanding of the nature of theinvention, several embodiments of the gas delivery system will now bedescribed in detail, by way of example only, with reference to theaccompanying drawings in which:

FIG. 1 is a functional block diagram of one embodiment of a gas deliverysystem according to the invention;

FIG. 2 is a schematic diagram of a second embodiment of a gas deliverysystem according to the invention; and,

FIG. 3 is a section view through the cylinder head of an internalcombustion engine illustrating an embodiment of a gas delivery systemsimilar to that of FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The gas delivery system illustrated in FIG. 1 for a gas fuelled-internalcombustion engine (not shown) comprises gas fuel delivery means 10 fordelivering a controlled amount of gaseous fuel to a region adjacent asource of ignition in the engine. This region in the engine comprises apre-combustion zone 12 and a combustion zone 14 in communication witheach other. The gas fuel delivery means 10 comprises first and seconddelivery means 16, 18 for delivering gaseous fuel to the pre-combustionzone 12 and the combustion zone 14 respectively, and further comprisesgas control means 20, 22 for controlling the relative proportions ofgaseous fuel delivered by said first and second delivery means 16, 18respectively. The first and second delivery means 16, 18 deliver gaseousfuel from gas supply 24, and may take the form of, for example, gassupply lines. The pre-combustion gas control means 20 and combustion gascontrol means 22 may take the form of, for example, gaseous fuelinjectors.

In this embodiment, the pre-combustion zone 12 is located in immediateproximity to the ignition source 26 which may be, for example, a sparkplug, to facilitate ignition of gaseous fuel in the pre-combustion zone12. The ignition source 26 is under the control of ignition controlmeans 28.

Air is delivered to the combustion zone 14 from air supply 30 via airdelivery means 32 and combustion air control means 34. In thisembodiment, air is not delivered directly to the pre-combustion zone 12.Combustion air control means controls the quantity, temperature andpressure of combustion air prior to delivery to the combustion zone 14via combustion air delivery means 36, which is commonly an enginemanifold and inlet valve system. The products of combustion exit theengine from the combustion zone 14 via an exhaust delivery system 38,which commonly takes the form of an exhaust valve system and manifoldsystem to an exhaust disposal means 40. An exhaust emissions analyser 42analyses the exhaust emissions and generates a signal which istransmitted via signal delivery line 44 to an Engine Management System(EMS) 46.

The EMS 46 provides overall management of the engine operation and bothmonitors and supervises the pre-combustion gas control means 20,ignition control means 28, combustion gas control lines 22 andcombustion air control means 34 via signal delivery lines 48, 50, 52 and54 respectively. The signal delivery means 44, 48, 50, 52 and 54 maytake the form of, for example, electrical cables, optical fibres or anyother suitable signal transmission medium. The EMS 46 may also monitorand/or supervise the control of other operating parameters of the enginesuch as engine speed, position of the throttle, engine phase, dynamicengine load and battery voltage. Each of these operating parameters mayhave some bearing on the ultimate control of the gas delivery systemdescribed above, however they have been omitted from FIG. 1 for claritysake. The exhaust emissions analyser 42 provides a feedback signal tothe EMS 46 and is derived from a measurement directly or indirectly ofthe exhaust gas contents of the engine for the presence of carbonmonoxides, hydrocarbons, nitrous oxides or other undesirable emissions.Such emissions give an indication of the extent and degree of combustionof gaseous fuel occurring within the engine and may be used by the EMS46 to adjust the relative proportions of gaseous fuel delivered to thepre-combustion zone 12 and/or the combustion zone 14, or the totalquantity of gas over time being supplied to the engine.

As noted above, combustion air is delivered directly to the combustionzone 14 only, however, during a compression stroke of the engine someair is communicated from the combustion zone 14 to the pre-combustionzone 12 via pre-combustion air supply path 56. In practice,pre-combustion zone 12 may be located in a separate pre-combustionchamber which is in direct communication through an orifice with acylinder of the engine, the combustion zone 14 being located in thecylinder above the piston. According to the present invention, duringoperation of the engine, the ignition source 26 is employed to initiateignition of gaseous fuel in the pre-combustion zone 12 where arelatively rich fuel to air ratio is present. Indeed, the ratio of fuelto air in the pre-combustion zone is selected so that ignition of thegaseous fuel is a certainty. The path of the resultant gases from theignition of gaseous fuel in pre-combustion zone 12 to the combustionzone 14 is shown as 58, and in practice would be through the sameorifice between the pre-combustion chamber and the engine cylinder. Thevolume of the pre-combustion zone 12 is designed to provide enoughchemical reaction via the path 58 of the resultant gases from thepre-combustion zone 12 to the combustion zone 14, to cause ignition ofthe gaseous fuel and air present in the combustion zone 14, which has alean fuel to air ratio. Thus, combustion in the combustion zone 14 canbe achieved with minimum gaseous fuel by initiating ignition of thegaseous fuel in the pre-combustion zone 12 in the manner describedabove.

In FIG. 2 a second embodiment of the gas delivery system according tothe invention is illustrated schematically, and comprises gas fueldelivery means 60 for delivering a controlled amount of gaseous fuel toa region adjacent a source 62 of ignition, for example, a spark plug inthe engine. In FIG. 2 only one cylinder 64 of the engine is illustratedshowing a piston 66 near the top of its compression stroke. The regionadjacent the ignition source 62 comprises a pre-combustion zone 68located within a pre-combustion chamber 70 and a combustion zone 72located within the cylinder 64 above the piston 66. The pre-combustionchamber 70 is in direct communication through an orifice 74 with thecylinder 64. The gas fuel delivery means 60 comprises first and secondgas delivery lines 76, 78 for delivering gaseous fuel to thepre-combustion zone 68 and combustion zone 72 respectively. The gasdelivery lines 76, 78 are supplied with gaseous fuel from a singlesolenoid actuated gas injector 80.

The gas fuel delivery means 60 further comprises gas control means inthe form of a gas flow valve 52 for controlling the relative proportionsof gaseous fuel delivered by the first and second delivery lines 76, 78respectively. In this embodiment, the gas flow valve 82 is a non-returnor one way valve which opens when the pressure within the pre-combustionchamber 70 is lower than the pressure within the gas delivery line 76,but which closes when the pressure within the pre-combustion chamber 70exceeds the pressure within the gas delivery line 76. Hence, during adownward stroke of piston 66 one way valve 82 allows the flow of gaseousfuel from the delivery line 76 into the pre-combustion chamber 70,however during a compression stroke of the piston 66 the one way valve82 closes to cue off the flow of gaseous fuel into the pre-combustionchamber 70. One way valve 82 also isolates the gas delivery line 76 fromthe gases produced as a result of combustion in the pre-combustion zone68 and combustion zone 72.

The relative proportions of gaseous fuel delivered to the pre-combustionzone 68 and the combustion zone 72 is largely controlled by the size ofan aperture provided within the one way valve 82 in its open condition.Typically, the size of the aperture in one way valve 82 is selected sothat between 1 percent to 10 percent of the gaseous fuel supplied fromthe gas injector 80 passes through the gas delivery line 76 to thepre-combustion zone 68, and the remainder of the gaseous fuel isdelivered by gas delivery line 78 to the combustion zone 72 withincylinder 64. Typically, the gas delivery line 78 delivers gas to thecombustion zone 72 via the engine manifold and inlet valve system. Inthis embodiment, the one way valve 82 is selected so that 4 percent ofthe gaseous fuel from the common injector 80 is delivered to thepre-combustion zone 68, however the exact relative proportions ofgaseous fuel delivered to the pre-combustion zone 68 and combustion zone72 respectively will depend on the operating characteristics of theparticular engine. From the above description, it will be apparent thatthe relative proportions of gaseous fuel delivered by the first andsecond gas delivery lines 76, 78 is fixed for this particular engine, asdetermined by the size of the aperture within one way valve 82. However,it is possible to arrange the gas fuel delivery means 60 so that therelative proportions of gaseous fuel delivered by the first and secondgas delivery lines 76, 78 can be varied, by for example, supplyinggaseous fuel to the delivery lines 76, 78 from separate gas injectors.The amount of gaseous fuel delivered by the gas injectors to therespective delivery lines 76, 78 could then be varied, for example,under the control of an engine management system.

The relative proportions of gaseous fuel delivered to the pre-combustionchamber and the cylinder for a particular engine can be determined asfollows. The objective is to achieve a gas/air ratio in thepre-combustion chamber (PCC) which will ignite easily from a spark plug.In one embodiment, natural gas is used as the gaseous fuel. Natural gasis composed mainly of methane and has a stoichiometric gas/air ratio of0.095 (9.5%). Ignition can still be achieved within the range ofapproximately 6.0 to 15.0% with a PCC, however most reliable ignition isachieved at near stoichiometric conditions.

There are many variables which contribute to the overall determinationof the desired quantity of gas that should be diverted to the PCC. Tofacilitate the determination, certain variables were fixed and theresultant relationship of gas/air ratio in the PCC as a function ofspark advance angle, and total gas quantity injected as a volumepercentage of the total cylinder volume was calculated. The compressionratio is fixed at a value determined by the mechanical and thermodynamicconsiderations that apply to the particular engine, due to its desiredoutput and gas quality. A low PCC/MCC is desirable to reduce the localthermal losses of the PCC, however the variability of the gas flowcontrol at the PCC will dictate a practical limit. The level ofgas/cylinder volume is dictated by the designed maximum absolute boostpressure, compression ratio and engine output requirements.

The following definitions and formula apply:

PCVFRN=Pre-chamber volume fraction-fraction of clearance volume occupiedby PCC.

PCGFRN=Pre-chamber gas fraction-fraction of gas injected into the PCC.

CR=Compression ratio. =Clearance vol+swept vol (100) Clearance vol

CV=Clearance volume =Swept vol (100) Compression ratio--1

GV=Gas volume injected by one injector

PCGV=Pre-chamber gas volume injected =GV×PCGFRN

PCV=Pre-chamber volume =CV×PCVFRN

MCGV=Main Chamber (or cylinder) gas volume =GV-PCGV

MCAV=Main chamber air volume =100-MCGV

MCGR=Main chamber gas ratio =MCGV/MCAV

THETA=Spark advance angle (0-1.57 Radians) ##EQU1##

A number of iterations can be performed by varying the PCC/MCC gas ratiountil near stoichiometric conditions are achieved for the complete rangeof envisaged spark advance settings. In one embodiment this processindicated that a PCGFRN of 0.04 (4%) achieved near stoichiometricconditions over a range of spark advance settings (THETA) of 0.0 to0.5236 radians. PCVFRN was set at 0.08, CR at 12.5 and CV of 8.6956512(cylinder of volume size 100 units).

The operation of the gas delivery system illustrated in FIG. 2 issimilar to that of the system illustrated in FIG. 1. During the airinlet stroke of piston 66 a mixture of gaseous fuel and air aredelivered to the combustion zone 72, while simultaneously gaseous fuelonly is delivered to the pre-combustion zone 68 via one way valve 82.During a compression stroke of the piston 66 some of the air fuelmixture within combustion zone 72 communicates into pre-combustionchamber 70 via orifice 74, and ignition of gaseous fuel is initiatedwithin the pre-combustion zone 68 by ignition source 62. By this stage,one way valve 82 has closed. Due to the relatively rich fuel to airratio within pre-combustion zone 68 ignition can be readily achieved anda chemical reaction occurs via the path of the resultant gases throughorifice 74 from the pre-combustion zone 68 to the combustion zone 126 toproduce ignition of the lean gaseous fuel/air mixture in the combustionzone 72. Hence, the gas fuelled internal combustion engine can beoperated within a lean burn regime without fear of engine misfire.

In FIG. 3 the cylinder head 84 of a compression ignition engine isillustrated in section view. The compression ignition engine of FIG. 3has been converted to a gas fuelled, spark ignition engine, andincorporates an embodiment of a gas delivery system according to theinvention, which is similar to that of FIG. 2. A casing 86 is fixedwithin a bore 88 provided in the cylinder head 84, so that cooling water89 circulating within the cylinder head also cools the casing 86. Casing86 defines a pre-combustion chamber 90 therein, and is manufactured froma material having a high thermal conductivity so that some of the heatgenerated within the pre-combustion chamber 90 is conducted through thewalls of the casing 86 to the cooling water 89. The pre-combustionchamber 90 defines a pre-combustion zone 92 therein, and is in directcommunication with a cylinder 94 of the engine, within which acombustion zone 96 is located.

Casing 86 also houses therein an ignition source 98 (spark plug), and aone way valve 100. Gaseous fuel is delivered to the one way valve 100via a gas delivery line 102, similar to that of FIG. 2.

The operation of the gas delivery system of FIG. 3 is similar to that ofFIG. 2, and will not be described again in detail here. The one wayvalve 100 controls the relative proportions of gaseous fuel delivered tothe pre-combustion zone 92 and combustion zone 96, via respective gasdelivery lines and a common gas injector (not illustrated). Thecombustion process is initiated by spark plug 98 within thepre-combustion zone 92, and the resultant effects of gaseous fuelignition in the pre-combustion zone 92 are directed via orifice 93 tothe combustion zone 96 to achieve combustion of the remaininggaseous-fuel.

Obviously, the arrangement of the pre-combustion chamber 90 within thecylinder head 84 of the engine may vary considerably from thatillustrated in FIG. 3 depending on the type of engine, particularlywhere the pre-combustion chamber is incorporated within the cylinderhead at the time of manufacture of the engine, rather than as a resultof a conversion of a compression ignition engine to a spark ignition gasfuelled or dual fuel engine. In a dual fuel engine, the gas deliverysystem according to the invention can also be used to deliver pilot fuelto the engine.

It will be apparent from the above description that the gas deliverysystem according to the invention has significant advantages over priorart systems which rely on a gas carburettor to supply the appropriateair/fuel mixture to the engine. The relative proportions of gaseous fueldelivered to the pre-combustion zone and the combustion zone can beaccurately controlled so that combustion in the combustion zone can beachieved with minimum gaseous fuel without fear of engine misfire.Combustion in the pre-combustion zone produces a flame front and radicalmolecules which will easily and quickly ignite the relatively leanmixture in the combustion zone, thus resulting in high engineefficiencies, i.e., more combustion close to piston top dead centre(TDC) and lower losses due to dissociation and thermal considerations,which both increase rapidly as the peak cycle temperature increases.Furthermore, emissions of carbon monoxide, hydrocarbons and nitrousoxides can be lowered due to the overall lean combustion process. Thesimplicity and elegance of the gas delivery system according to theinvention enables it to be readily incorporated in a conventional engineand/or a conventional engine can be easily converted to a gas fuelledengine.

Furthermore, now that preferred embodiments of the gas delivery systemhave been described in detail, it will be obvious to persons skilled inthe mechanical arts, that numerous modifications and variations may bemade to the illustrated embodiments, in addition to those alreadydescribed, without departing from the basic inventive concepts. Forexample, the relative proportions of gaseous fuel delivered to thepre-combustion zone and the combustion zone respectively, may bedetermined by the relative diameters of the respective gas deliverylines. All such variations and modifications are to be considered withinthe scope of the present invention, the nature of which is to bedetermined from the foregoing description and the appended claims.

The claims defining the invention are as follows:
 1. A gas deliverysystem for a gas fuelled internal combustion engine, the systemcomprising:gas fuel delivery means for delivering a controlled amount ofgaseous fuel to a region adjacent a source of ignition, said regioncomprising a pre-combustion zone located in a separate pre-combustionchamber which is located in immediate proximity to said ignition sourceand which is in direct communication through an orifice with acombustion zone located in a cylinder of the engine, said gas fueldelivery means comprising: a single gas injector which is common to saidpre-combustion zone and said combustion zone for said cylinder of theengine; a first gas delivery line for delivering a controlled amount ofgaseous fuel to said pre-combustion zone and a second gas delivery linefor delivering a controlled amount of gaseous fuel to said combustionzone, both from said single gas injector common to said pre-combustionzone and said combustion zone for said cylinder, and gas control meansfor controlling the relative proportions of gaseous fuel delivered bysaid first gas delivery line and said second gas delivery line from saidsingle gas injector common to said pre-combustion zone and saidcombustion zone for said cylinder, whereby, in use, combustion in saidcombustion zone can be achieved with minimum gaseous fuel by initiatingignition of gaseous fuel in said pre-combustion zone.
 2. A gas deliverysystem as claimed in claim 1, wherein said gas control means comprises agas flow valve for controlling the quantity of gaseous fuel delivered tosaid pre-combustion zone.
 3. A gas delivery system as claimed in claim2, wherein said gas flow valve is a one way valve for controlling thequantity of gaseous fuel delivered via said first gas delivery line tosaid pre-combustion zone.
 4. A gas delivery system as claimed in claim1, wherein the relative proportions of gaseous fuel delivered by saidfirst gas delivery line and by said second gas delivery line is fixedfor a particular engine.
 5. A gas delivery system as claimed in claim 4,wherein the relative proportion of gaseous fuel delivered by said firstgas delivery line may be in the range of 1 to 10 percent of the totalquantity of gaseous fuel delivered to said region by said gas fueldelivery means.
 6. A gas delivery system as claimed in claim 3, whereinsaid ignition source and one way valve are housed in a casing mountedabove said cylinder in a cylinder head of the engine, and wherein saidpre-combustion chamber is defined by the casing and is manufactured froma material having a thermal conductivity adapted to allow heat generatedwithin the pre-combustion chamber to be conducted through a wall of thecasing to cooling water in the cylinder head.
 7. A gas delivery systemas claimed in claim 3, wherein gaseous fuel only is delivered via saidfirst gas delivery line to said pre-combustion zone, and wherein duringa compression stroke some of an air-fuel mixture within said combustionzone communicates into said pre-combustion zone via said orifice so thatignition of gaseous fuel can be initiated within said pre-combustionzone by said ignition source.
 8. A gas delivery system as claimed inclaim 1, wherein said gas control means comprises a gas flow valve forcontrolling the quantity of gaseous fuel delivered to said combustionzone.
 9. A gas delivery system as claimed in claim 1, wherein said gascontrol means comprises a gas flow valve for controlling the quantity ofgaseous fuel delivered to said pre-combustion zone and to saidcombustion zone.